To the knowledge of the applicant, the drafting system is an important part of a spinning machine and its performance directly affects the quality of resultant yarns of the spinning machine. The drafting systems of existing spinning machines are mostly three-roller drafting mechanisms, and the typical three-roller drafting mechanisms include the ordinary three-roller-double-apron structure and the three-roller-four-apron structure which can achieve a draft ratio of 80 to 100.
The ordinary three-roller-double-apron structure is divided into a front draft zone and a rear draft zone. Since only simple roller drafting is used in the rear draft zone, the middle friction field is weak and the draft ratio is generally less than 1.4. It is difficult for such a structure to improve the total draft ratio of spinning machines. The total draft ratio of such a structure is small and is in the range of 20 to 50.
To solve the problem about the low draft ratio in the rear draft zone of the above-mentioned structure, the Chinese invention patent (patent number 961.07383.7 and authorized publication number CN1056205C) and the Chinese utility model patent (patent number 01243901.0 and patent publication number CN2495663Y) respectively disclose a three-roller-three-apron drafting mechanism and a three-roller-four-apron drafting mechanism. These drafting mechanisms can overcome the defect about a low drafting ratio in the rear draft zone of a three-roller drafting mechanism and can increase the maximum total draft ratio to 100 and 150, respectively.
The above-mentioned drafting mechanisms all adopt a three-roller-double-draft-zone structure, wherein the front draft zone is the main draft zone and bears the major drafting responsibility; the rear draft zone prepares for the front draft zone, pre-drafts and combs fed strands. Since fed roving strands have a certain twist, the rear draft zone needs to untwist fed strands so that they are more orderly before going to the front draft zone and they can be fully drafted in the front draft zone. On the basis of this, the draft ratio in the rear draft zone is heavily restricted. As a result, the maximum total draft ratio of the whole draft mechanism is restricted and a further breakthrough cannot be made.
To solve the problem, the Chinese utility model (patent number 03217782.8 and authorized publication number CN2623707Y) discloses a four-roller-four-apron drafting mechanism for a spinning machine. Such a drafting mechanism has a front draft zone, a middle draft zone, and a rear draft zone, and the maximum total draft ratio is up to 200. To further improve the mechanism, the Chinese invention patent (patent number 200710143612.6 and authorized publication number CN100545331C) discloses a four-roller drafting device for a ring spinning machine. The four-roller drafting device effectively solves the problem about hairiness during twisting of yarns. However, the four-roller drafting technique in the two representative patents is formed by simply adding one line of rollers on the basis of the existing three-roller drafting technique. No breakthrough has been made in its drafting principle. The maximum total draft ratio is already bottlenecked and cannot further be broken through. In addition, the four-roller structure already reaches the limit of the number of lines of rollers. So far, a drafting mechanism with at least five lines of rollers has never been put into practice successfully. Therefore, a drafting technique which can break through the existing drafting principle and can further increase the maximum total draft ratio urgently needs to be developed.